Life histories of North American woodpeckers

by Arthur Cleveland Bent

A nesting tree that may be regarded as typical stood in a dense forest, entirely of hardwood—maples, elms, and yellow birches—on the plain of a high and ancient beach of Lake Superior, cut through by a mountain stream, and about a hundred yards from the water. It was the smooth and barkless stub of a dead elm, about 45 feet high and having a girth, breast-high, of 76 inches. The hole was smooth and white, and the wood was still firm. The stub stood well shaded beneath the living trees. A few flecks of morning sunlight fell upon its eastern face; but throughout the greater part of the day it remained in shadow. It had been the woodpeckers’ nesting place certainly for four years. The highest hole seemed to be the oldest—in the south face and near the top. The uppermost 6 feet of the stub had since become weathered and checked and manifestly unsuitable. Next, on the north face, there was an old and black-looking hole about 36 feet up. The third and lowest hole was in the east face and about 25 feet up; and, lastly, there was the hole of the year, 34 feet up and also in the east face.

The chamber within is capacious and is ordinarily of conical form, tapering slightly from a low domed roof downward to a bowllike bottom. There may be a slight bulging of the walls below a narrowed median portion. The depth may vary from 10 to 24 inches (extreme figures of 6 and 26 have been recorded). The average of 15 measurements is 19 inches. The entrance hole leads to the upper widest portion, and there the chamber is 7 or 8 inches across. The distance from the outer surface of the hole of the tree to the remote wall of the chamber is about 11 inches. The entrance passageway about 2 inches inward is ridged across, and from this median ridge the floor of the passageway slopes downward, both inwardly and outwardly, and this outward slope forms the bevel already mentioned. The bowl at the bottom is 6 or 6½ inches across. In a specimen before me as I write, the wall of the chamber below the entrance hole is 4 inches thick. The ridge across the floor of the entrance passageway is rounded. Its crest is 2¼ inches inward from the outer surface of the tree trunk, and the vertical depth of the outward bevel is 2 inches. All the surfaces of the cavity are neatly and uniformly chiseled. Along the sides of the entrance passageway extend in parallel curves the tool marks of the bird’s beak. No nesting materials are brought in. A feather or two will be the only trace of occupancy remaining after the young are flown. In some though not in all cases it is possible for a man to thrust in his arm and reach the bottom of the chamber.

As a general rule, certainly, a new cavity is drilled for every brood. Such exceptions as have been recorded have explanation in human interference. Samuel Scoville, Jr. (1920), quotes Richard C. Harlow to the effect that but once in his experience had a second use of a nesting cavity occurred. Afterward Mr. Harlow said to me in conversation that even in that instance the cavity had been deepened before it was used for a second time. The only other instance that has come to my attention is one recorded by Morrell (1901) in which a single cavity was used three times—in 1895, in 1897, and in 1898. In preparation for the third nesting the cavity had been deepened by three inches. This nesting was in “a small patch of good sized trees * * * separated [from] the main growth by cutting,” and it may be supposed that the woodpeckers had been unduly limited in the choice of nesting sites. In both cases the birds were subject to the disturbance of persistent egg-collecting. It stands to reason that, in avoidance of parasites, the practice should have evolved of drilling a fresh cavity for each brood.

Mr. Harlow (1914) found that in one instance the drilling of the nesting cavity was in progress in March and was continued “all during March and April.” The female worked alone, and the male continued near by. This nest, an unusually early one, contained, on April 30, three eggs. In the Northern States the eggs commonly are laid early in May. Incubation continues, according to Burns (1915), for 18 days. The young leave the nest about the middle of June.

The range of date in nesting is illustrated by two records that come from Centre County, Pa. (Scoville, 1920; Burleigh, 1931). One is of a set of eggs that hatched on May 11. These eggs must have been laid before April 23. The other record is of a set collected May 11 and found to be practically fresh. The interval at which these two cases stand apart is about 25 days. Scoville (1920) quotes Harlow, a collector whose experience was chiefly in Centre County, to the effect that “May 10 is the standard date for a full clutch of eggs.”

Records of nests are at hand from Maine, New Hampshire, New York, Pennsylvania, Ohio, and Wisconsin. The total body of data, however, is small; and it is not possible to discover what the difference may be in mean nesting date from south to north within the region covered. There is a record from Maine, for instance (Morrell, 1901), of a set of eggs found to be heavily incubated as early as May 13.

Eggs.—The eggs are white, with a gleaming smoothness and translucence of shell. They rest at the bottom of the cavity, on the bare bed of finely splintered wood. Three eggs often complete the set, but more commonly four. Of 17 recorded sets, 4 are sets of three, and 13 are sets of four. Some of the earlier writers (Wilson, for instance, 1811) said the number of eggs might be five or even six; but no specific record of so large a number has been found. The eggs are of ovate outline.

The measurements of 51 eggs average 33.16 by 25.21 millimeters; the eggs showing the four extremes measure 38.2 by 27.1, 30.2 by 25.2, and 33.05 by 23.75 millimeters.

There are cases on record in which a pair of these birds, robbed of their eggs, have laid again (and in the same cavity); with this qualification, there is but a single brood in a season.

Young.—In a particular instance; which I take to be typical, of a nesting (in northern Fulton County, Pa.) I found the male to be no less attentive than the female to the duties of incubation and nurture. In one respect, indeed, the male seemed to be the more attentive, for on both of the two occasions when I had opportunity to observe—once shortly before, the other shortly after the hatching of the eggs—it was the male bird who at sunset retired within the hole and who at sunrise the following morning appeared from within. And I mention this the more confidently since I find chance confirmation in the narrative of another observer, Morrell (1901), and since like observations have been made upon other species of woodpeckers—upon the flicker, for instance, and upon the ivory-billed woodpecker (Allen, 1937).